a. Field of the Invention
The instant invention relates to a method and apparatus for complex impedance compensation, and, in another aspect, a method and apparatus for using complex impedance measurements including a phase angle to detect tissue morphology.
b. Background Art
Many applications have been proposed for the use of a measured impedance of tissue, for example cardiac tissue, and not surprisingly a number of approaches for making such impedance measurements have also been proposed. One procedure where an impedance is measured and used is in cardiac catheter ablation.
As general background, there are a number of methods used for ablation of desired areas, including, for example, radio frequency (RF) ablation. RF ablation is accomplished by transmission of radio frequency energy to a desired target area through an electrode assembly to ablate tissue at the target site. RF ablation may generate significant heat if not controlled. It is therefore known to provide an RF ablation generator with certain feedback features, such as temperature and impedance. To provide such feedback for the physician/clinician's use during the procedure, conventional RF ablation generators are typically configured to measure and display a magnitude of a complex impedance |Z| intended to represent the impedance of the patient's tissue near and around the ablation electrode. To make the impedance measurement, conventional generators use one tip conductor (i.e., one lead through the catheter to the ablation tip electrode) and one ground conductor (i.e., one lead from a ground patch or the like back to the generator). The frequency of the source used to make the impedance measurement is generally the ablation energy source frequency, which can typically vary from 50 kHz to 500 kHz depending on the ablation generator. Such impedance measurements are commonly used to assess tissue heating and tissue-electrode contact levels. However, one shortcoming in the art is that such measurements are subject to variation due to factors unrelated to the condition of the tissue (i.e., non-physiologic changes). For example, coiling of an ablation generator cable during the procedure can increase its inductance and thus alter the impedance measurement, resulting in an inaccurate reading that is not completely indicative of the actual tissue condition. This scenario is particularly problematic where a phase angle of the complex impedance of cardiac tissue is sought, since this parameter is fairly low to begin with (<15°) for the frequencies typically used for RF ablation.
Another shortcoming is that conventional approaches for complex impedance measurements do not provide any intelligence as to tissue morphology, but rather appear to relate to tissue heating and electrode-to-tissue contact levels. An improvement to conventional approaches has been proposed which uses a phase measurement to assess the contact level for the catheter electrode. Tissue is more capacitive and resistive than myocardium due to the cell structure. Therefore, compared with blood, the measured impedance becomes more capacitive and the phase angle becomes more negative when the electrode is in contact with myocardium.
There is therefore a need to minimize or eliminate one or more of the problems set forth above.